1. Field of the Invention
The present invention relates to a mat with low X-ray absorption used in an X-ray diagnostic apparatus, e.g., an X-ray CT (computerized tomographic) apparatus.
2. Description of the Related Art
In X-ray diagnostic apparatuses, various types of mats positioned on table tops have been utilized, on which a biological body under medical examination, for instance, a patient is mounted during medical examination, as represented in, e.g., U.S. Pat. No. 4,312,912 to Tamura, issued on Jan. 26, 1982.
In FIG. 1, there is shown a conventional mat 1, as a cross-sectional view, used in the X-ray diagnostic apparatus (not shown in detail).
More specifically, the mat 1 is employed for X-ray radiography or fluoroscopy equipped with an X-ray detecting means such as an image intensifier (not shown). The mat 1 is mounted on the table couch (not shown) made of wood, and the biological body (not shown) under X-ray examination is mounted on this mat 1.
As is known in the art, it is preferable to reduce the X-ray absorption coefficients of all materials or constructive members other than the object to be X-ray-examined, through which X-rays pass, as small as possible, and also to achieve uniform lower X-ray absorption amounts thereof. Accordingly, as a material for this mat 1, a foaming plastic plate 2 with an elastic characteristic having a thickness of approximately 30 mm, for instance, polyurethane foam is generally used because of a small X-ray absorption coefficient thereof.
In the X-ray radioscopy or fluoroscopy, a preselected chemical agent such as an X-ray contrast medium is utilized. To this end, the outer surfaces of the mat 1 is covered with a waterproof sheet 3 whose X-ray absorption coefficient is greater than that of the foaming plastic plate 2 with elastic characteristics, and two end portions of this waterproof sheet 3 are overlap-jointed with each other at both sides of the foaming plastic plate 2, as shown in FIG. 1.
In general, an X-ray projection direction of such an X-ray diagnostic apparatus is substantially perpendicular to a major plane of the mat 1 on which a biological body under medical examination is mounted. There are some possibilities that a diagnostic portion of the biological body under medical examination might exceed the edge portions of this mat 1, depending upon the body size. When such an exceeding diagnostic portion is imaged by the X-ray radiation, one X-ray beam "Xa" will pass through a portion "1a" near a center portion of the mat 1, having small aluminium equivalent for X-ray attenuation, e.g., 0.2 mm, whereas another X-ray beam "Xb" will pass through only a portion of the waterproof sheet 3 having a length of 30 mm at an end portion "1b" of this mat 1. Accordingly, since aluminium equivalent for X-ray attenuation of this end portion "1b" becomes larger than 1.5 mm, namely becomes 7 times higher than the aluminium equivalent of the above-described portion "1a", this end portion "1b" appears as a white line in a resultant X-ray image, which may give adverse influences to the X-ray diagnosis.